In vertebrates these muscles are also known as involuntary and visceral; involuntary because the activities of these muscles are not under the control of will and, thus, are controlled by the autonomic nervous system, and visceral because these muscles are found predominantly in visceral tissues such as in the walls of the four great tracts of hollow organs—the circulatory, the respiratory, the alimentary and the urinogenital.
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They are also found in the interior of the eye (the ciliary muscles and the muscle in the iris) in skin (the pilomotor, contraction of which erects the hair and produces goose bumps) and in the ducts of the glands.
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Smooth muscles contract slowly and latently where as striated muscles contract rapidly and almost immediately at threshold.
These muscles no doubt display considerable tonicity, a state in which at a given time some myofibrils are in a state of contraction and others are in a state of relaxation.
These are further distinguished from striated muscles in being innervated exclusively by the autonomic nervous system.
Smooth muscles exhibit longitudinal striations in their ultra- structure but cross or transverse striations are lacking completely.
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Smooth muscles occur as sheets of fibres, bundles of fibres or as single isolated fibres and exhibit considerable diversity in their arrangement and position in the body as well as in their mode of behaviour. Two major groups, based on physiological function, are recognized.
The unitary muscles occur in sheets or layers and are characterized by their ability to contract spontaneously.
This activity originates within the muscle since pace-makers from which spontaneous contractile activity arises are found within the muscles.
The pace-makers are not restrictively localized as they are in the cardiac muscles of vertebrate hearts but are scattered diffusely throughout vast expanses of tissue.
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The pace-maker cells respond to stretch by generating action potentials. Unitary muscles are especially responsive to the mechanical stimulus of stretch but their activity is modulated and coordinated by neural influences.
The muscles of gastro-intestinal tract and the ureters are the good examples of such types of muscles.
The multiunit muscles are those smooth muscles which do not contract spontaneously; which are unresponsive to stretch and which have a multiple innervation by motor nerves so that each cell may be innervated.
Some smooth muscles such as muscles of urinary bladder resemble both unitary and multiunit muscles in that they may react spontaneously as well as reacting to motor nerve impulses.
The smooth muscle fibres are innervated by the vegetative nervous system, viz., sympathetic and parasympathetic nervous system.
The primary function of these muscles is to contract rhythmically and the precise cause of the same is not known.
These muscles show rhythmic contraction even when they are denervated. Sympathetic nervous system stimulates the contraction of smooth muscles in some organs, while it depresses their activity in others.
Similar role is played by parasympathetic nervous system but sympathetic and parasympathetic nervous systems are opposite in function.
By rhythmic contraction and relaxation these muscles in the wall of alimentary canal and in the urinary and reproductive organs propel the contents of these organs forward (peristalsis).
In the walls of the blood vessels they govern the amount of blood passing through a blood vessel and thus its distribution.
These muscles in the form of strong circular band or ring (sphincter) control the opening and closing of a tube orifice.